If you get disconnected by “Nice try | You’ve been kicked from the server.” message or see “Sent trololo and dropped client” on the server, that is because there is a constant variable named OFFICIAL, which defaults to true when the build is in Debug configuration.
So, as far as I understand, it prevents people from joining to the developer’s server when the developer actively debugs the code. And the link JetFox gave as server files are in Debug configuration. To get Release configuration, download server source files and install Visual Studio with .NET Desktop Environment included. Unpack source files into a temporary folder (you might keep it if you want to), then open “dmultiplayersrv.sln” (you might not see .sln if showing extensions is disabled) using Visual Studio. If you have no idea how you can open it, just start up the Visual Studio and choose File -> Open -> Project/Solution from the toolbar. There, you should see Debug as a dropdown list in the toolbar. Change it to Release and then you can build it by choosing Build -> Build Solution from the same toolbar.
Built files should be inside “.\dmultiplayersrvgui\bin\Release”, which is relative to the unpack location. Use those server files instead and it’ll work as expected.